1. Field of the Invention
The present invention relates to the field of sheet processing, and more particularly to the field of calendering a paper sheet to achieve a desired sheet finish and caliper.
2. Description of the Related Art
One of the aspects by which sheet materials are graded is the "finish" of the sheet surface. For example, paper may be categorized into various grades having different degrees of smoothness or gloss for various applications. Sheet smoothness and gloss are collectively known as sheet "finish." Although smoothness and gloss are important characteristics of most papers, the term "smoothness" is most often used in connection with uncoated paper sheet, while the term "gloss" usually refers to the shininess of coated paper sheet, such as that often used in magazines.
Bulk paper is frequently produced in a continuous sheet which may be wound in a roll. The paper roll may have a dimension in the cross-direction (i.e., across the width of the sheet) of 25 feet or more. The continuous sheet may then be unrolled and cut into individual sheets of the desired size. The consistency of the surface finish between the individual sheets depends upon the uniformity of the finish of the original bulk paper roll. Thus, it is important to have a uniform finish across the width and along the length of the continuous bulk paper roll.
Another characteristic by which sheet materials are graded is sheet thickness or "caliper." Bulk paper production typically involves a calendering process which includes pressing the paper sheet material between a plurality of calender rolls to obtain the desired sheet characteristics. For example, subjecting a paper sheet to the calendering process can change its caliper as well as its finish. Sheet finish, including gloss and smoothness, may be conventionally controlled by applying steam to the surface of the paper sheet, followed by pressing the sheet between a series of calender rolls. Typically, the series of calender rolls is arranged in a stack, which may consist of alternating hard, polished steel rolls and soft, resilient rolls made of cotton or polymers. A typical series of such hard and soft rolls is known as a "supercalender".
The paper absorbs the heat and moisture of the steam, and paper fibers at the sheet surface are softened. As the polished steel roll comes into contact with the paper surface that has been treated with steam, the paper surface is smoothed and pressed flat by the pressing and rubbing action of the hard steel roll against an adjacent soft roll, thereby producing a smooth or glossy finish on the surface of the paper. This process is similar to treating a laundered shirt with a steam iron and ironing board to removing wrinkles from the cloth. The degree of smoothness, or gloss, is dependent upon the amount of heat, moisture and pressure applied to the sheet.
Unfortunately, a problem commonly associated with the use of steam treatments to create a desired finish is that the steam treatment used to affect the finish also produces a concurrent effect on the caliper profile of the paper. Specifically, the heat and moisture of the steam may penetrate the paper sheet and soften both its surface and core fibers. Subsequent action by the calender rolls, while smoothing the softened surface fibers and creating the desired finish, also simultaneously decreases the caliper of the sheet because the core fibers are also unintentionally softened. An increase in the smoothness or gloss of the paper surface may thus be "coupled" to a substantial decrease in sheet caliper. However, the desired caliper and surface finish could be obtained with greater predictability and precision if the two characteristics could be partially or completely "decoupled", (i.e., controlled independently).
A related problem associated with systems that regulate the amount of steam applied to different sections of the surface of the paper sheet lies in the fact that steam is used. Upon contact with the sheet, the condensing steam liberates a substantial amount of heat energy to the sheet. However, for saturated steam, the relationship between moisture and heat is fixed. That is, for a given volume and flow of steam applied to a sheet surface, there will be a fixed amount of available heat and water in the steam. Therefore, with steam, the amount of moisture, which primarily effects finish, is directly proportional to the amount of heat, which affects both the finish and sheet caliper. Thus, a paper mill operator's flexibility in producing a paper sheet of a desired finish and caliper may be extremely limited using conventional steam systems.